A number of minor improvements

Here are a number of small modifications to CL-CUDD to make it load smoothly.

I fixed the issue with PI by shadowing it in the package for the API.  I also refined the foreign library loader to handle Mac as well as Linux.

I think it would be possible to have ASDF automatically run SWIG to generate the cuddapi.lisp file, but haven't done that yet.

LMK if you are no longer interested in this library, and I will stop bothering you.